Magnetic devices and structures

ABSTRACT

Magnetic devices, magnetoresistive structures, and methods and techniques associated with the magnetic devices and magnetoresistive structures are presented. For example, a magnetic device is presented. The magnetic device includes a ferromagnet, an antiferromagnet coupled to the ferromagnet, and a nonmagnetic metal proximate to the ferromagnet. The antiferromagnet provides uniaxial anisotropy to the magnetic device. A resistance of the nonmagnetic metal is dependent upon a direction of a magnetic moment of the ferromagnet.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.12/553,619, filed on Sep. 3, 2009, the disclosure of which is fullyincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to magnetic devices,spintronics, memory and integrated circuits. More particularly theinvention relates to magnetoresistive structures (e.g., spin torquestructures) and devices including magnetoresistive random access memory(MRAM).

BACKGROUND OF THE INVENTION

Magnetoresistive random access memories (MRAMs) may combine magneticcomponents with standard silicon-based microelectronics to achievenon-volatile memories. For example, silicon based microelectronicsinclude electronic devices such as transistors (e.g., field effecttransistors or bipolar transistors), diodes, resistors, interconnect,capacitors or inductors. Other MRAMs may comprise magnetic componentswith other semiconductor components, for example, components comprisinggallium arsenide (GaAs), germanium or other semiconductor material.

An MRAM memory cell comprises a magnetoresistive structure that stores amagnetic moment that is switched between two directions corresponding totwo data states (“1” and “0”). In an MRAM cell, information is stored inmagnetization directions of a free magnetic layer. In a conventionalspin-transfer MRAM memory cell, the data state is programmed to a “1” orto a “0” by forcing a write current directly through the stack of layersof materials that make up the MRAM cell. Generally speaking, the writecurrent, which is spin polarized by passing through one layer, exerts aspin torque on a subsequent free magnetic layer. The torque switches themagnetization of the free magnetic layer between two stable statesdepending upon the polarity of the write current.

SUMMARY OF THE INVENTION

Principles of the invention provide, for example, magnetic devices, andtechniques and methods associated with magnetic devices.

For example, in accordance with one embodiment of the invention, amagnetic device is presented. The magnetic device includes aferromagnet, an antiferromagnet coupled to the ferromagnet, and anonmagnetic metal proximate to the ferromagnet. The antiferromagnetprovides uniaxial anisotropy to the magnetic device. A resistance of thenonmagnetic metal is dependent upon a direction of a magnetic moment ofthe ferromagnet.

In accordance with another embodiment of the invention, amagnetoresistive structure is presented. The magnetoresistive structurecomprises a ferromagnetic layer and an antiferromagnetic layer coupledto the ferromagnetic layer. A free side of the magnetoresistivestructure includes the ferromagnetic layer and the antiferromagneticlayer. The magnetoresistive structure further comprises a pinned layerand a nonmagnetic metallic layer at least partly between the free sideand the pinned layer. The antiferromagnetic layer provides uniaxialanisotropy to the free side. A resistance of the nonmagnetic metalliclayer is dependent upon a direction of a magnetic moment of theferromagnetic layer.

In accordance with an additional embodiment of the invention, amagnetoresistive memory device is presented. The magnetoresistive memorydevice includes the above magnetoresistive structure and stores at leasttwo data states corresponding to at least two directions of a magneticmoment.

In accordance with yet another embodiment of the invention, anintegrated circuit is presented. The integrated circuit includes theabove magnetoresistive structure and a substrate on which the pinnedlayer, the nonmagnetic metallic layer, the ferromagnetic layer and theferrimagnetic layer are formed.

In accordance with another additional embodiment of the invention, amethod for forming a magnetoresistive structure is presented. The methodincludes forming a ferromagnetic layer and an antiferromagnetic layercoupled to the ferromagnetic layer. A free side of the magnetoresistivestructure comprises the ferromagnetic layer and the antiferromagneticlayer. The method further includes forming a pinned layer and anonmagnetic metallic layer at least partly between the free side and thepinned layer. The antiferromagnetic layer provides uniaxial anisotropyto the free side. A resistance of the nonmagnetic metallic layer isdependent upon a direction of a magnetic moment of the ferromagneticlayer.

Compared to conventional spin-torque switched magnetoresistive devices,aspects of the invention reduce switching current and/or increaseswitching speed. An antiferromagnet provides additional anisotropy tomagnetic devices, for example, a free side antiferromagnetic layerprovides additional uniaxial anisotropy to magnetoresistive devicesenabling faster switching and/or lower switching currents.

These and other features, objects and advantages of the presentinvention will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a spin-torque magnetoresistive structure.

FIG. 2 illustrates a spin-torque structure comprising a free-sidebilayer, according to an embodiment of the present invention.

FIG. 3 illustrates a spin-torque structure comprising a free-sidebilayer and an alternate pinned side configuration, according to anembodiment of the present invention.

FIG. 4 illustrates writing a spin-torque structure, according to anembodiment of the present invention.

FIG. 5 illustrates a method for forming a magnetoresistive structure,according to an embodiment of the invention.

FIG. 6 is a cross-sectional view depicting an exemplary packagedintegrated circuit, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Principles of the present invention will be described herein in thecontext of exemplary magnetic, magnetoresistive and spin-torque switcheddevices and method for forming and use of such device. It is to beunderstood, however, that the techniques of the present invention arenot limited to the devices and method shown and described herein. Ratherembodiments of the invention are directed to techniques for providinguniaxial anisotropy to magnetic devices, and to techniques for reducingswitching current and/or increasing switching speed in spin-torqueswitched devices. Although embodiments of the invention may befabricated in or upon a silicon wafer, embodiments of the invention canalternatively be fabricated in or upon wafers comprising othermaterials, including but not limited to gallium arsenide (GaAs), indiumphosphide (InP), etc. Although embodiments of the invention may befabricated using the materials described below, alternate embodimentsmay be fabricated using other materials. The drawings are not drawn toscale. Thicknesses of various layers depicted by the drawings are notnecessarily indicative of thicknesses of the layers of embodiments ofthe invention. For the purposes of clarity, some commonly used layers,well known in the art, have not been illustrated in at least some of thedrawings of FIGS. 1-4, including, but not limited to, metal contactinglayers, protective cap layers, seed layers, and an underlying substrate.The substrate may be a semiconductor substrate, such as silicon, or anyother suitable structure.

The term magnetic device, as used herein, is a device that comprises amagnet. A magnet may be, for example, a ferromagnet, an antiferromagnet,or a nanomagnet. A ferromagnetic layer is an example of a ferromagnet.An antiferromagnetic layer is an example of an antiferromagnet. Examplesof magnetic devices include, but are not limited to, a magnetic tunneljunction, a magnetoresistive structure, a magnetoresistive random accessmemory, a spin-torque-switched device and a spin-valve.

Ferromagnetic materials exhibit parallel alignment of atomic magneticmoments resulting in relatively large net magnetization even in theabsence of a magnetic field. The parallel alignment effect only occursat temperatures below a certain critical temperature, called the Curietemperature. In ferromagnets, two nearby magnetic dipoles tend to alignin the same direction because of the Pauli principle: two electrons withthe same spin cannot also have the same “position”, which effectivelyreduces the energy of their electrostatic interaction compared toelectrons with opposite spin.

The atomic magnetic moments in ferromagnetic materials exhibit verystrong interactions produced by electronic exchange forces and result ina parallel or anti-parallel alignment of atomic magnetic moments.Exchange forces can be very large, for example, equivalent to a field onthe order of 1000 Tesla. The exchange force is a quantum mechanicalphenomenon due to the relative orientation of the spins of twoelectrons. The elements iron (Fe), nickel Ni, and cobalt (Co) and manyof their alloys are typical ferromagnetic materials. Two distinctcharacteristics of ferromagnetic materials are their spontaneousmagnetization and the existence of magnetic ordering temperatures (i.e.,Curie temperatures). Even though electronic exchange forces inferromagnets are very large, thermal energy eventually overcomes theexchange and produces a randomizing effect. This occurs at a particulartemperature called the Curie temperature (T_(c)). Below the Curietemperature, the ferromagnet is ordered and above it, disordered. Thesaturation magnetization goes to zero at the Curie temperature.

Antiferromagnetic materials are materials having magnetic moments ofatoms or molecules, usually related to the spins of electrons, align ina regular pattern with neighboring spins, on different sublattices,pointing in opposite directions. Generally, antiferromagnetic order mayexist at sufficiently low temperatures, vanishing at and above a certaintemperature, the Néel temperature. Above the Néel temperature, thematerial is typically paramagnetic. When no external magnetic field isapplied, the antiferromagnetic material corresponds to a vanishing totalmagnetization. In a magnetic field, ferrimagnetic-like behavior may bedisplayed in the antiferromagnetic phase, with the absolute value of oneof the sublattice magnetizations differing from that of the othersublattice, resulting in a nonzero net magnetization.

Antiferromagnets can couple (e.g., exchange-couple) to ferromagnets, forinstance, through a mechanism known as exchange anisotropy (for,example, when a ferromagnetic film is either grown upon theantiferromagnet or annealed in an aligning magnetic field) causing thesurface atoms of the ferromagnet to align with the surface atoms of theantiferromagnet. This provides the ability to pin the orientation of aferromagnetic film. The temperature at or above which anantiferromagnetic layer loses its ability to pin the magnetizationdirection of an adjacent ferromagnetic layer is called the blockingtemperature of that layer and is usually lower than the Néeltemperature.

Saturation magnetization (M_(s)) of a magnetic material is the magneticfield of the magnetic material wherein an increase in an externallyapplied magnetic field H does not significantly increase themagnetization (i.e., magnetic field B of the magnetic material) of themagnetic material further, so the total magnetic field B of the magneticmaterial levels off Saturation magnetization is a characteristicparticularly of ferromagnetic materials. In fact, above saturation, themagnetic field B continues increasing, but at the paramagnetic rate,which can be, for example, 3 orders of magnitude smaller than theferromagnetic rate seen below saturation.

A nanomagnet is a sub-micrometric system that presents spontaneousmagnetic order (magnetization) at a zero applied magnetic field. Thesmall size of nanomagnets typically prevents the formation of magneticdomains. The magnetization dynamics of sufficiently small nanomagnets atlow temperatures, typically single-molecule magnets, presents quantumphenomena such as macroscopic spin tunneling. At higher temperatures themagnetization undergoes random thermal fluctuations (superparamagnetism)which can, for example, limit the use of nanomagnets for permanentinformation storage. Examples of nanomagnets include grains offerromagnetic metals (e.g., iron, cobalt, and nickel) andsingle-molecule magnets. Some nanomagnets comprise transition metal(e.g., titanium, vanadium, chromium, manganese, iron, cobalt or nickel)or rare earth (e.g., gadolinium, europium or erbium) magnetic atoms.Nano magnets can be formed, for example, as magnetic layers such asferromagnetic layers and antiferromagnetic layers.

Magnetic anisotropy is the direction dependence of magnetic propertiesof a material. A magnetically isotropic material has no preferentialdirection for a magnetic moment of the material in a zero magneticfield, while a magnetically anisotropic material will tend to align itsmoment to an easy axis. There are different sources of magneticanisotropy, for example: magnetocrystalline anisotropy, wherein theatomic structure of a crystal introduces preferential directions for themagnetization; shape anisotropy, when a particle is not perfectlyspherical, the demagnetizing field will not be equal for all directions,creating one or more easy axes; stress anisotropy, wherein tension mayalter magnetic behavior, leading to magnetic anisotropy; uniaxialanisotropy, that is, anisotropy along a single axis (e.g., a single easyaxis); and exchange anisotropy that occurs when antiferromagnetic andferromagnetic materials interact. The Anisotropy field (H_(k)) may bedefined as the weakest magnetic field which is capable of switching themagnetization of the material from the easy axis. In multilayerstructures, the effective magnetic anisotropy energy can bephenomenologically split into two components, a volume component (volumeanisotropy) and an interface contribution.

Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistanceeffect observed in certain structures, for example, structurescomprising two magnetic layers (e.g. ferromagnetic or ferrimagneticlayers) with a nonmagnetic layer between the two magnetic layers. Themagnetoresistance effect manifests itself as a significantly lowerelectrical resistance of the nonmagnetic layer, due to relatively littlemagnetic scattering, when the magnetizations of the two magnetic layersare parallel. The magnetizations of the two magnetic layers may be madeparallel by, for example, placing the structure within an externalmagnetic field. The magnetoresistance effect further manifests itself asa significantly higher electrical resistance of the nonmagnetic layer,due to relatively high magnetic scattering, when the magnetizations ofthe two magnetic layers are anti-parallel. Because of anantiferromagnetic coupling between the two magnetic layers, themagnetizations of the two magnetic layers are anti-parallel when thestructure is not at least partially within an external magnetic field.

The term nonmagnetic metal, as used herein, means a metal that is notmagnetic including not ferromagnetic and not antiferromagnetic.

Tunnel magnetoresistance (TMR) is a magnetoresistive effect that occursin magnetic tunnel junctions (MTJs). A MTJ is a component consisting oftwo magnets separated by a thin insulator. If the insulating layer isthin enough (typically a few nanometers), electrons can tunnel from onemagnet into the other. Since this process is forbidden in classicalphysics, TMR is a strictly quantum mechanical phenomenon.

The term proximate or proximate to, as used herein, has meaninginclusive of, but not limited to, abutting, in contact with, andoperatively in contact with. In particular and with respect to magneticcoupling, proximate or proximate to includes, but is not limited to,being operatively magnetically coupled. The term abut(s) or abutting, asused herein, has meaning that includes, but is not limited to, beingproximate to.

A spin-transfer-switched device is a device that includes a two terminalmagnetic junction that can be switched by passing an electric currentthrough the spin-transfer-switched device. A spin-transfer-switcheddevice is a spin-torque structure and may be, for example, amagnetoresistive structure or device. The magnetic junction is either atunnel junction or a spin-valve. The electric current becomesspin-polarized in the spin-transfer-switched device. Conservation ofspin-angular momentum is a mechanism behind a current-driven magneticreversal process. In the spin-transfer-switched device there are usuallytwo magnetic layers separated by a barrier layer such as a tunnelbarrier layer or a conducting nonmagnetic barrier layer. One of themagnetic layers is “fixed” by its relatively large layer thickness or byexchange-coupled magnetic pinning. The other magnetic layer (i.e., freemagnetic layer) is free or rotatable. The free layer usually has atleast two stable magnetic directions. For digital applications the twostable magnetic directions correspond to “0” and “1” data states. Aswitch between the two stable magnetic directions can be accomplished bydriving the junction with a current exceeding a certain switchingthreshold-current, Ic. The direction of the current determines which ofthe two stable magnetic directions the final state becomes. Thespin-transfer-switched device is, for example, formed with a lateralsize at or below 100 nanometers (nm). A spin-transfer-switched devicewith a lateral size well below 100 nm is more operationally efficientthat a spin-transfer-switched device having a lateral size above 100 nm.The spin-transfer-switched device could extend magnetoresistive randomaccess memory (MRAM) into the fabrication technologies having dimensionsof thirty-two nanometers and below.

It is desirable to reduce the switching threshold-current, Ic, and toswitch between the stable magnetic directions (i.e., magnetic reversal)as fast as possible. Lowering Ic and faster switching may beaccomplished by, for example, reducing saturation magnetization of afree-side. A low magnetic moment of the free-side allows for fastswitching because spin torque induced switching conserves angularmomentum, and magnetic moment is proportional to the angular momentum.Electrons have spin-angular momentum. The lower the total magneticmoment of the free-side, the less spin-angular momentum needed to induceswitching and, therefore, the fewer the electrons needed to induceswitching In this way, Ic is reduced and, for a given switching current,switching occurs faster.

A free-side comprising a material having a low saturation magneticmoment also reduces Ic because Ic is determined, at least in part, by atotal anisotropy field of the free-side. For a thin film structuredfree-side, the total anisotropy field of the free-side is usuallydominated by a thin film shape-related easy-plane anisotropy fieldhaving a saturation magnetization 4\pi Ms that could be well into manykilo-oersteds. A reduction of Ms causes a reduction of Ic.

However, for commonly used simple ferromagnetic materials, such as thinfilms comprising an alloy comprising cobalt and iron (CoFe) or an alloycomprising cobalt, iron and boron (B) (i.e., CoFeB), there is a limit tothe reduction of Ms because, for commonly used simple ferromagneticmaterials, the intrinsic magnetic anisotropy is minimal and/oruncontrolled. The uniaxial anisotropy necessary for maintaining thermalstability of a nanomagnet, with respect to the data state stored withinthe nanomagnet, is controlled mostly by the shape demagnetization of thenanomagnet, which requires a sizeable Ms. The use of shape-controlledmagnetic anisotropy in thin films also results in a very largeeasy-plane anisotropy which amounts to an anisotropy field having asaturation magnetization 4\pi Ms that could be well into manykilo-oersteds for the commonly used simple ferromagnetic materials. Thiseasy-plane anisotropy is what currently limiting the further reductionof Ic.

The requirement for reducing Ms, in order to reduce Ic, is therefore incompetition with the requirement of good thermal stability for dataretention. Good thermal stability for data retention usually requiresthe uniaxial anisotropy to be, for example, 40 kilo-telsa (kT) to 60 kTin order to ensure ten year data retention.

The choice of free-side materials at the barrier layer to free-sideinterface is limited because of the required magnetotransport properties(e.g., large magnetoresistance and appropriate resistance-area product)of the junction comprising the barrier layer. The magnetotransportproperties also, at least in part, determine the efficiency of thespin-transfer-switched device and the value of Ic.

Therefore, it is desirable to have a free-side that: i) derives uniaxialanisotropy from sources other than shape, ii) keeps the total magneticmoment, and therefore Ms, at a minimum, and iii) preserves thespin-polarization properties at the barrier layer to free-side interfacein order to maximize the efficiency of spin-torque switching (e.g.,maximum switching speed, minimum switching current).

An aspect of the invention is that the intrinsic anisotropy energy of athin antiferromagnet, for example, a thin antiferromagnetic layer(AFML), provides additional uniaxial anisotropy to a thin ferromagnetfor example, a thin ferromagnetic layer (FML). The thin antiferromagnetand the thin ferromagnet may be a nano-structured antiferromagnet (i.e.,a nanoantiferromagnet) and a nano-structured ferromagnet (i.e., ananoferromagnet), respectively. A magnetic device comprising the AFML(antiferromagnet) and the FML (ferromagnet), for example as a bi-layer,is provided with uniaxial anisotropy by the AFML.

A corresponding embodiment of the invention is a spin-torque-switchablemagnetic tunnel junction (MTJ) device comprises the thin FML and thethin AFML within a composite free-side structure of the device. Thecomposite free-side structure, comprising the ferromagnet and theantiferromagnet, may be considered a composite nanomagnet, a compositethin-film structure, and a by-layer. The FML is stronglyexchange-coupled to the AFML. Both the FML and the AFML are formed tohave substantially the same laterally dimensions, for example, less thanone hundred and twenty nanometers. The volume anisotropy of the AFMLprovides additional thermal activation barrier height for the compositefree-side structure. The additional thermal activation barrier height islimited by the exchange coupling energy between the FM and AFM.

The thin FML comprises a high-spin-polarization ferromagnetic material,for example, iron (Fe) or an alloy of iron and cobalt (FeCo). The thinAFML comprises, for example, an alloy of iridium and Manganese (IrMn).Fe is adapted for interfacing with a (100) magnesium oxide (MgO) tunnelbarrier.

FIG. 1 illustrates a spin-torque structure 100. The spin-torquestructure 100 may be, at least part of, a spin-torque transfermagnetoresistive structure or spin-torque magnetoresistive random accessmemory (MRAM) having two terminals. The spin-torque structure 100comprises a free-side 110 comprising a free ferromagnetic layer 111,tunnel barrier layer 120, and pinned-side 130 comprising a pinnedferromagnetic layer 131 and a pinned-side antiferromagnetic layer 132. Amagnetic tunnel junction comprises the tunnel barrier layer 120 betweenthe free-side 110 and the pinned-side 130. The direction of the magneticmoment of the pinned ferromagnetic layer 131 is fixed in direction(e.g., pointing to the right) by the pinned-side antiferromagnetic layer132. A current, exceeding a switching threshold-current Ic, passed downthrough the tunnel junction makes magnetization of the freeferromagnetic layer 111 parallel to the magnetization of the pinnedferromagnetic layer 131, e.g., pointing to the right (down is in thevertical direction from the top to the bottom of FIG. 1). A current,exceeding Ic, passed up through the tunnel junction makes themagnetization of the free ferromagnetic layer 111 anti-parallel to themagnetization of the pinned ferromagnetic layer 131, e.g., pointing tothe left. A smaller current, less Ic, through the device 100, passing upor passing down, is used to read the resistance of the device 100, whichdepends on the relative orientations of the magnetizations of the freeferromagnetic layer 111 and the pinned ferromagnetic layer 131. Thesmaller current is not intended to change, and should be small enough sothat it does not change, the magnetization of the free ferromagneticlayer 111.

FIG. 2 illustrates a spin-torque structure 200 comprising a free-sidebilayer, according to an embodiment of the present invention. Thespin-torque structure 200 may be, for example, a spin-transfer-switcheddevice. The spin-torque structure 200 comprises an MgO tunnel barrierlayer 120 and a pinned-side 130 comprising a pinned-sideantiferromagnetic layer 132 and a pinned ferrimagnetic layer 131.

The spin-torque structure 200 further comprises a free side 210comprising a FML 216 and an AFML 215. The FML 216 is exchange-coupled tothe AFML 215. The FML 216 is in direct contact with (e.g., abuts, isproximate to) the AFML 215 and with the tunnel barrier layer 120. Thepinned ferromagnetic layer 131 abuts the tunnel barrier layer 120 andthe pinned-side antiferromagnetic layer 132.

Alternately, the spin-torque structure 200 may comprise a metallicspacer layer in place of the tunnel barrier layer 120. The spin-torquestructure comprising the metallic spacer layer exhibits giantmagnetoresistance as previously described. The resistance of thenonmagnetic metallic layer depends upon the relative orientation of thetwo abutting or proximate layers, i.e., the pinned ferrimagnetic layer131 and the FML 216. A first resistance of the nonmagnetic metalliclayer corresponds to the direction of the magnetic moment of the FML 216being parallel to a direction of a magnetic moment of the pinnedferromagnetic layer 131, and a second resistance of the nonmagneticmetallic layer corresponds to the direction of the magnetic moment ofthe FML 216 being anti-parallel to the direction of the magnetic momentof the pinned ferromagnetic layer 131. Resulting from the giantmagnetoresistance effect, the first resistance is lower than the secondresistance.

As an example, in the spin-torque structure 200, the FML 216 comprises a5 angstrom thick layer (or between three and seven angstroms) of analloy of iron and cobalt (FeCo) and the AFML comprises a 15 angstromthick layer (or between ten and twenty angstroms) of IrMn. The free sidemay further comprise additional layers, for example, a 50 angstrom (orbetween thirty to seventy angstroms) thick layer of ruthenium (Ru) (notshown), a nonmagnetic transition metal, abutting the AFML 215 and a topmetal layer (not shown) coupled to a top contact 219. The pinned-side130 may further comprise additional layers, for example, a bottom metallayer (not shown) abutting the pinned-side antiferromagnetic layer 132and coupled to a bottom contact 239. A magnetic tunnel junction (MTJ)comprises the tunnel barrier layer 120 between the pinned-side 130 andthe free-side 210 (e.g., between the pinned ferrimagnetic layer 131 andthe FML 216).

In the spin-torque structure 200, the FML 216 interfaces with the tunnelbarrier layer 120. The function of the FML 216 interfacing with thetunnel barrier layer 120 is to provide spin-polarization in the densityof state for the MTJ. The interface and the related function dictate aspecific type of band-structure of the FML 216 matched to theband-structure of tunnel barrier layer 120. Consequently, thecrystalline orientation and symmetry of FML 216 has to match to thecrystalline orientation and symmetry of tunnel barrier layer 120. In thespin-torque structure 200, MgO of the tunnel barrier layer 120 is in a(100) texture (e.g., MgO was grown or otherwise formed in a (100)texture). The FeCo of the FML 216 is grown or otherwise formed inbody-centered cubic (bcc) symmetry and grown or formed on the MgOepitaxially, thus, at least preserving a grain-to-grain epitaxialrelationship of the FeCo of the FML 216 with the abutting MgO of thetunnel barrier layer 120.

The function of the AFML 215 is to provide an adequate amount ofmagnetic anisotropy (e.g., uniaxial magnetic anisotropy) without addingsignificant amounts of total magnetic moment into the compositefree-side structure.

The FML 216 and the AFML 215, in the composite free-side structure, areexchange-coupled magnetically across the interface between the FML 216and the AFML 215 (FML/AFML interface). The exchange coupling energy mustbe strong relative to the anisotropy energy of the AFML 215 in order toensure coherent rotation, under spin-torque excitation, of the magneticmoments of both the FML 216 and the AFML 215. The maximum amount ofuniaxial anisotropy energy the AFML 215 can provide is therefore limitedby, and to the value of, the FML/AFML interface exchange couplingenergy. Consequently, the exchange coupling energy of the exchangecoupling between the AFML and the FML is greater than energy associatedwith the uniaxial anisotropy of the AFML.

FIG. 3 illustrates a spin-torque structure 300 comprising a free-sidebilayer, according to another embodiment of the invention. Thespin-torque structure 300 comprises an MgO-barrier based,spin-torque-switched magnetic tunnel junction formed with lateraldimensions below, for example, 100 nm (nanometers), or less than 120 nm.The spin-torque structure 300 comprise a bottom contact 339 coupled to abottom metal electrode 338 abutting and coupled to a ferromagnetic firstlayer 337 comprising CoFe, a nonmagnetic second layer 336 comprising Ruabutting the ferromagnetic first layer 337, a ferromagnetic third layer331 comprising CoFeB abutting the nonmagnetic second layer 336, a tunnelbarrier fourth layer 120 comprising MgO, abutting the ferromagneticthird layer 331, a FML 216 that is approximately five angstroms thickcomprising CoFe abutting the tunnel barrier fourth layer 120, an AFML215 that is approximately fifteen angstroms thick comprising IrMn andproximate to the FML 216, and a top contact 319 coupled to a top metalelectrode 318 abutting and coupled to the AFML 215. A free-side 210comprises the AFML 215 abutting and exchange-coupled to the FML 216. Thefree-side 210 is configured and functions similarly to the free-side 210of the spin-torque structure 200 of FIG. 2. A pined side 330 comprisesthe ferromagnetic first layer 337, the nonmagnetic second layer 336 andthe ferromagnetic third layer 331. The ferromagnetic third layer 331 isa pinned layer. The direction of the magnetic moment of theferromagnetic third layer 331 is fixed in direction or pinned (e.g.,pointing to the right), at least in part, by the ferromagnetic firstlayer 337.

Alternately, the spin-torque structure 300 may comprise a metallicspacer layer in place of the tunnel barrier fourth layer 120. Thespin-torque structure comprising the metallic spacer layer exhibitsgiant magnetoresistance as previously described. The resistance of thenonmagnetic metallic layer depends upon the relative orientation of thetwo abutting or proximate layers, i.e., the ferrimagnetic third layer331 and the FML 216. A first resistance of the nonmagnetic metalliclayer corresponds to the direction of the magnetic moment of the FML 216being parallel to a direction of a magnetic moment of the ferromagneticthird layer 331, and a second resistance of the nonmagnetic metalliclayer corresponds to the direction of the magnetic moment of the FML 216being anti-parallel to the direction of the magnetic moment of theferromagnetic third layer 331. Resulting from the giantmagnetoresistance effect, the first resistance is lower than the secondresistance.

The tunnel barrier layer 120 may comprise, for example, MgO (e.g., alayer comprising MgO). In the embodiments shown in FIGS. 2 and 3, thetunnel barrier layer 120 is an example of a nonmagnetic spacer layer andalso a tunnel junction layer. Other embodiments, having amagnetoresistance signal due to giant magnetoresistance, may include anonmagnetic metallic layer as a nonmagnetic spacer layer in place of thetunnel barrier layer 120. Embodiments comprising the nonmagneticmetallic layer operate, for example, during reading or writing, in asimilar way as embodiments comprising the tunnel barrier layer, althoughthe underlying physics of the magnetoresistances differ between thetunnel barrier layer (tunneling magnetoresistance) and the nonmagneticmetallic layer (giant magnetoresistance). The nonmagnetic metallic layermay comprise, for example, copper (Cu), gold (Au), or Ru. Embodimentscomprising the nonmagnetic metallic layer are sometimes calledspin-valves.

A spin-torque device, such as an MRAM memory or MRAM memory cell,according to an embodiment of the invention comprises, for example, thespin-torque structure 200, the spin-torque structure 300 or thespin-torque structure 400. An MRAM, comprising one or more of the MRAMmemory cells, may further comprise other electronic devices orstructures such as electronic devices comprising silicon, a transistor,a field-effect transistor, a bipolar transistor, ametal-oxide-semiconductor transistor, a diode, a resistor, a capacitor,an inductor, another memory device, interconnect, an analog circuit anda digital circuit. Data stored within the MRAM memory cell correspondsto the direction of a magnetic moment in the FML 216 layer and/or theAFML 215 layer.

In the embodiments of FIGS. 1 and 2, the pinned side 130 comprises apinned ferromagnetic layer 131 and a pinned-side antiferromagnetic layer132 abutting and exchange-coupled to the pinned ferromagnetic layer 131.Although the pinned side 130 comprises the layers shown in FIGS. 1 and2, the invention is not so limited; other arrangements of the pinnedside 130 are known in the art and may be used in other embodiments ofthe invention.

The pinned ferromagnetic layer 131 may comprise, for example, ananti-parallel (AP) layer comprising a 2 nanometer (nm) thick layercomprising a first alloy of cobalt and iron (CoFe), a 0.8 nm ruthenium(Ru) layer, and another 2 nm thick layer comprising a second alloy ofcobalt and iron (CoFe). Alternately, the pinned ferromagnetic layer 131may comprise a simple pinned layer, for example, a 3 nm thick layer ofan alloy of cobalt and iron (CoFe).

The pinned-side antiferromagnetic layer 132 is strongly exchange-coupledto the pinned ferromagnetic layer 131 pinning the pinned ferromagneticlayer 131. The pinned-side antiferromagnetic layer 132 is used to pinthe pinned ferromagnetic layer 131 to a particular alignment.

The pinned-side antiferromagnetic layer 132 may comprise, for example,an alloy of manganese (Mn) such as an alloy comprising iridium andmanganese (IrMn), an alloy comprising platinum and manganese (PtMn), analloy comprising iron and manganese (FeMn), or an alloy comprisingnickel and manganese (NiMn). Alternately, the pinned-sideantiferromagnetic layer 132 may comprises, for example, differentantiferromagnetic materials.

FIG. 4 shows the write operation of the spin-torque structure 400,according to an embodiment of the invention. The spin-torque structure400 comprised the spin-torque structure 200 with a write currentapplied. Writing, in one case, is accomplished by an upwards writecurrent 410A, comprising a flow of electrons driven vertically throughthe spin-torque structure 400. The direction of the arrows on the heavyvertical lines points in the direction of electron flow. To change thedata state of the spin-torque structure 400, the write current switchesthe magnetic moment of the FML 216. Because the AFML 215 is stronglyexchange-coupled to the FML 216, the magnetic moment of the AFML 215 isalso switched. If a magnetic moment 421 of the pinned ferromagneticlayer 131 points, for example, to the left, the electrons flowing withinthe upwards current 410A will be spin-polarized to the left andtherefore place a torque on the FML 216 to switch a magnetic moment 422Aof the free FML 216 to the left. Correspondingly, due to the exchangecoupling between the AFML 215 and the FML 216, a magnetic moment 423A ofthe AFML 215 may also be switched to the left. If the data state alreadycorresponded to the data state that otherwise would be induced by theupwards write current 410A, the magnetic moment 422A of the FML 216 andthe magnetic moment 423A of the AFML 215 were already set to the leftand will not be switched by the upwards write current 410A.

Conversely, if the flow of electrons is in the opposite direction(downward) as in the downward write current 410B, the electrons will bespin-polarized to the right, and a magnetic moment 422B of the FML 216will be switched to the right when changing the data state.Consequently, a magnetic moment 423B of the AFML 215 will also beswitched to the right. If the data state already corresponded to thedata state that otherwise would be induced by the downward write current410B, the magnetic moment 422B of the FML 216 and the magnetic moment423B of the AFML 215 were already set to the right and will not beswitched by the downwards write current 410B.

The direction of the magnetic moment 421 of the pinned ferromagneticlayer 131, for example, is set using a high-temperature anneal in anapplied magnetic field.

Consider reading the spin-torque structure 200. In one embodiment, aread current, less than the write current and below the switchingthreshold-current, k, is applied to read the resistance of the tunnelbarrier layer 120. The read current is applied across the spin-torquestructure 200 to flow through the spin-torque structure 200 from top tobottom or from bottom to top. The resistance of the tunnel barrier layer120 depends on the relative magnetic orientation (direction of magneticmoment) of the FML 216. If the magnetic orientations are parallel, theresistance of the tunnel barrier layer 120 is relatively low. If themagnetic orientations are anti-parallel, the resistance of the tunnelbarrier layer 120 is relatively high. As previously stated, theresistance of the tunnel barrier layer 120 is due to tunnelingmagnetoresistance, and the resistance of a nonmagnetic metal layer thatmay be used as a nonmagnetic spacer layer in place of the tunnel barrierlayer 120 is due to giant magnetoresistance. Measuring the voltageacross the spin-torque structure 200, corresponding to the applied readcurrent, allows for calculation of the resistance across the spin-torquestructure 200 according to ohms law. Because the resistance of thetunnel barrier layer 120 dominates the series resistance of the layerswithin the spin-torque structure 200, the resistance of the tunnelbarrier layer 120 is obtained, to some degree of accuracy, by measuringthe resistance of the spin-torque structure 200. In an alternate methodof reading, a read voltage is applied across the spin-torque structure200 and a current is measured from which the resistance of thespin-torque structure 200 is calculated.

Read and write operations of the spin-torque structure 300 are similarto the read and write operations described above for the spin-torquestructure 200.

FIG. 5 illustrates a method 500 for forming a spin-torque structure,according to an embodiment of the invention. For example, thespin-torque structure comprises the spin-torque structure 200 or an MRAMmemory cell. The steps of method 500 may occur in orders other than thatillustrated.

The first step 510 comprises forming a pinned-side antiferromagneticlayer, for example the pinned-side antiferromagnetic layer 132.

The second step 520 comprises forming a pinned ferromagnetic layer, forexample the pinned ferromagnetic layer 131. The pinned-sideantiferromagnetic layer is exchange-coupled and abutting the pinnedferromagnetic layer.

The third step 530 comprises forming a tunnel barrier layer. Forexample, the tunnel barrier layer comprises the tunnel barrier layer120. The tunnel barrier layer abuts the pinned ferromagnetic layer.

The fourth step 540 comprises forming a free-side ferromagnetic layer,for example, the FML 216. The free-side ferromagnetic layer abuts thetunnel barrier layer.

The fifth step 550 comprises forming a free-side antiferromagneticlayer, for example, the AML 215. The free-side antiferromagnetic layeris exchange-coupled to and abuts, or is proximate to, the free-sideferromagnetic layer.

According to an alternate method, the third step 530 comprises forming anonmagnetic metal layer instead of the tunnel barrier layer, wherein thepinned ferromagnetic and the free-side ferromagnetic layers abuts thenonmagnetic metal layer.

According to yet another alternate method, the first step 510 and thesecond step 520 are replaced by an alternate step of forming a pinnedside which may comprise one or more layers different from thecombination of the pinned-side antiferromagnetic layer and the pinnedferromagnetic layer. For example, the alternate step may compriseforming a pinned side (e.g., the pinned-side 330) comprising aferromagnetic layer (e.g., the ferromagnetic first layer 337), a spacerlayer (e.g., the nonmagnetic spacer layer 336) and another ferromagneticlayer (e.g., the ferromagnetic third layer 331).

FIG. 6 is a cross-sectional view depicting an exemplary packagedintegrated circuit 600 according to an embodiment of the presentinvention. The packaged integrated circuit 600 comprises a leadframe602, a die 604 attached to the leadframe, and a plastic encapsulationmold 608. Although FIG. 6 shows only one type of integrated circuitpackage, the invention is not so limited; embodiments of the inventionmay comprise an integrated circuit die enclosed in any package type.

The die 604 includes a structure described herein according toembodiments of the invention and may include other structures orcircuits. For example, the die 604 includes at least one magneticdevice, magnetoresistive, spin-torque structure or MRAM according toembodiments of the invention, for example, the spin-torque structures200, 300 or 400 or embodiments formed according to the method of theinvention (e.g., the method of FIG. 5). For example, the otherstructures or circuits may comprise electronic devices comprisingsilicon, a transistor, a field-effect transistor, a bipolar transistor,a metal-oxide-semiconductor transistor, a diode, a resistor, acapacitor, an inductor, another memory device, interconnect, an analogcircuit and a digital circuit. The spin torque structure or MRAM may beformed upon or within a semiconductor substrate, the die also comprisingthe substrate. Specifically, the substrate underlies, supports or is aplatform upon which, the spin torque structure or MRAM is formed, forexample, the pinned side 130, the tunnel barrier layer 120, the FML 216and the AFML are formed.

An integrated circuit in accordance with the present invention can beemployed in applications, hardware and/or electronic systems. Suitablehardware and systems for implementing the invention may include, but arenot limited to, personal computers, communication networks, electroniccommerce systems, portable communications devices (e.g., cell phones),solid-state media storage devices, functional circuitry, etc. Systemsand hardware incorporating such integrated circuits are considered partof this invention. Given the teachings of the invention provided herein,one of ordinary skill in the art will be able to contemplate otherimplementations and applications of the techniques of the invention.

Although illustrative embodiments of the invention have been describedherein with reference to the accompanying drawings, it is to beunderstood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may bemade therein by one skilled in the art without departing from the scopeof the appended claims.

What is claimed is:
 1. A magnetic device comprising: a ferromagnetlayer; an antiferromagnet layer coupled to the ferromagnet layer; and anonmagnetic metal proximate to the ferromagnet layer; wherein theantiferromagnet layer provides uniaxial anisotropy to the ferromagnetlayer, wherein an exchange coupling energy between the antiferromagnetlayer and the ferromagnet layer is greater than energy associated withthe uniaxial anisotropy to ensure concurrent rotation of magneticmoments of both the ferromagnet layer and the antiferromagnet layerunder spin-torque excitation; and wherein a resistance of thenonmagnetic metal is dependent upon a direction of the magnetic momentof the ferromagnet layer.
 2. The magnetic device of claim 1, wherein avolume anisotropy of the antiferromagnet layer provides a thermalactivation harrier for the magnetic device.
 3. The magnetic device ofclaim 1, wherein the ferromagnet layer is a nanoferromagnet.
 4. Amagnetoresistive memory device comprising: a ferromagnetic layer; anantiferromagnetic layer coupled to the ferromagnetic layer, wherein afree side of the magnetoresistive memory device comprises theferromagnetic layer and the antiferromagnetic layer; a pinned layerhaving a magnetic moment in a fixed direction; and a nonmagneticmetallic layer between the free side and the pimped layer; wherein theantiferromagnetic layer provides uniaxial anisotropy to theferromagnetic layer of the free side, wherein an exchange couplingenergy between the antiferromagnetic layer and the ferromagnetic layeris greater than energy associated with the uniaxial anisotropy to ensureconcurrent rotation of magnetic moments of both the ferromagnetic layerand antiferromagnetic layer under spin-torque excitation; wherein aresistance of the nonmagnetic metallic layer is dependent upon adirection of the magnetic moment of the ferromagnetic layer in relationto the fixed direction of the magnetic moment of the pinned layer; andwherein the magnetoresistive memory device stores at least two datastates corresponding to at least two directions of the magnetic momentof the ferromagnetic layer.
 5. The magnetoresistive memory device ofclaim 4, wherein the pinned layer comprises a pinned ferromagnetic layerand a pinned-side antiferromagnetic layer exchange-coupled to the pinnedferromagnetic layer.
 6. An integrated circuit comprising: aferromagnetic layer; an antiferromagnetic layer coupled to theferromagnetic layer, wherein the ferromagnetic layer and theantiferromagnetic layer comprise a free side of a magnetoresistivestructure; a pinned layer having a magnetic moment in a fixed direction;a nonmagnetic metallic layer between the free side and the pinned layer;and a substrate on which the pinned layer, the nonmagnetic metalliclayer, the ferromagnetic layer and the antiferromagnetic layer areformed; wherein the antiferromagnetic layer provides uniaxial anisotropyto the ferromagnetic layer of the free side, wherein an exchangecoupling energy between the antiferromagnetic layer and theferromagnetic layer is greater than energy associated with the uniaxialanisotropy to ensure concurrent rotation of magnetic moments of both theferromagnetic layer and the antiferromagnetic layer under spin-toqueexcitation; and wherein a resistance of the nonmagnetic metallic layeris dependent upon a direction of the magnetic moment of theferromagnetic layer in relation to the fixed direction of the magneticmoment of the pinned layer.
 7. The integrated circuit of claim 6,wherein the pinned layer comprises a pinned ferromagnetic layer and apinned-side antiferromagnetic layer exchange-coupled to the pinnedferromagnetic layer.
 8. A method for forming a magnetoresistivestructure, the method comprising the steps of: forming a ferromagneticlayer; forming an antiferromagnetic layer coupled to the ferromagneticlayer, wherein a free side of the magnetoresistive structure comprisesthe ferromagnetic layer and the antiferromagnetic layer; forming apinned layer having a magnetic moment in a fixed direction; and forminga nonmagnetic metallic layer between the free side and the pinned layer;wherein the antiferromagnetic layer provides uniaxial anisotropy to theferromagnetic layer of the free side, wherein an exchange couplingenergy between the antiferromagnetic layer and the ferromagnetic layeris greater than energy associated with the uniaxial anisotropy to ensureconcurrent rotation of magnetic moments of both the ferromagnetic layerand the antiferromagnetic layer under spin-torque excitation; andwherein a resistance of the nonmagnetic metallic layer is dependent upona direction of a magnetic moment of the ferromagnetic layer in relationto the fixed direction of the magnetic moment of the pinned layer. 9.The method of claim 8, wherein forming the pinned layer comprisesforming a pinned ferromagnetic layer and forming a pinned-sideantiferromagnetic layer exchange-coupled to the pinned ferromagneticlayer.